® International Journal of Plant Developmental Biology ©2010 Global Science Books Floral Organ Determination and Ontogenetical Patterns during Angiosperm Evolution Claudia Erbar* Heidelberg Institute of Plant Sciences – Biodiversity and Plant Systematics, University of Heidelberg, Im Neuenheimer Feld 345, D-69120 Heidelberg, Germany Correspondence : * [email protected] ABSTRACT Since the late 1980’s/early 1990’s enormous progress had been made in understanding the genetic and molecular regulation of flower development. The genetic ABCDE model describes five classes of genes that are responsible for the specification of floral organ identity in a combinatorial manner. The molecular quartet model advances the genetic ABCDE model by describing the presumed interactions between floral MADS-domain proteins. Although the basic developmental program appears to be quite conserved, in non-core eudicots modifications such as “sliding-boundary” and “fading borders” models had to be established. The genetic models proposed as yet predict the organ quality, but do not explain how the number of organs or the spatial pattern in which organ primordia appear (e.g. spiral or whorled) is regulated. With Apiaceae and Brassicaceae two families are presented which contrary to their uniform flower construction show fairly diverse patterns in organ initiation. Cleomaceae, sister to Brassicaceae, are even more diverse as regards stamen number as well as initiation patterns. The other members of the core Brassicales add further diversity to the androecial initiation pattern. Multistaminate androecia stand for a further interesting aspect as the stamen primordia are initiated either spirally directly on the floral apex or on so-called primary androecial primordia (fascicle primordia) in centrifugal or centripetal succession. Since the identity of floral organs is strictly dependent on the activity of the MADS-box genes, duplication and diversification within these genes must have been key processes in flower evolution. Hence, insights into the phylogeny of the floral homeotic genes may help to better understand the evolution of flowers (“evo-devo”). The unique nectary organs of the Ranunculaceae are presented as example for duplication and new functions in the B class genes. _____________________________________________________________________________________________________________ Keywords: ABCDE model, floral evolution, floral organ determination, flower development, flower ontogeny, ontogenetical patterns CONTENTS INTRODUCTION.......................................................................................................................................................................................... 1 MODIFICATIONS OF THE CLASSICAL ABCDE MODEL OF FLORAL ORGAN DETERMINATION ................................................ 3 DETERMINATION OF ORGAN IDENTITY IN MONOCOTS................................................................................................................... 3 DETERMINATION OF ORGAN IDENTITY IN NON-CORE EUDICOTS................................................................................................ 5 ORGAN QUALITY VERSUS ORGAN POSITION AND SEQUENCE OF ORGAN INITIATION........................................................... 6 THE ORIGIN OF THE ANGIOSPERM FLOWERS AND THEIR ENORMOUS DIVERSITY.................................................................. 9 REFERENCES............................................................................................................................................................................................. 13 _____________________________________________________________________________________________________________ INTRODUCTION several key questions remain. For example, what is the genetic basis of the different evolutionary innovations? Shortly after their “sudden” appearance, the angiosperms An extant complete angiosperm flower is composed of diversified quickly and had an explosive evolutionary the perianth (undifferentiated or differentiated in sepals and success and it is not surprising that Charles Darwin (in his petals), the microsporangia bearing, pollen producing sta- letter to Joseph Dalton Hooker, 22 July 1879; edited as mens (androecium) and the carpels (gynoecium) enclosing letter 395 by Francis Darwin and Albert C. Seward in 1903) the ovules (megasporangia), all arising from the floral axis referred to the origin and rapid radiation of the angiosperms of determinate growth. There are about 260,000-300,000 as an “abominable mystery” (but see on this topic Friedman, species of extant angiosperms with an enormous diversity 2009, who explicates Darwin’s thoughts as referring to the of size, shape, number of organs, contributing to flower possibility that evolution could be both rapid and poten- complexity. tially even saltional). Stuessy (2004) tried to resolve part of Genetic studies of homeotic1 mutants in Arabidopsis the problems by proposing the “transitional-combinational thaliana (Brassicaceae; see Fig. 1B) and Antirrhinum majus theory” for the origin of the angiosperms. This theory sug- (Plantaginaceae, formerly Scrophulariaceae; see Fig. 1C) gests that the angiosperms evolved slowly from seed ferns have shown that during the formation of the flower the in the Jurassic beginning first with the carpel, followed later determination of organ identity is controlled by homeotic by double fertilization, and lastly by the appearance of genes. The different analyses have led to the classical ABC flowers, a process that may have taken more than 100 model of Coen and Meyerowitz (1991), in which three million years (Stuessy 2004). Only the final combination of regions in a floral primordium are the domains of action of all three important features provided the opportunity for explosive evolutionary diversification, especially in res- 1 This term is derived from "homeosis". It refers to the wrong position of a ponse to selection from insect pollinators and predators as 1floral This organ:term is A derived floral fromorgan "homeosis". is found in It a refers place to where the wrong organs position of another of a well as in compatibility and breeding systems. However, floraltype are organ: normally A floral found, organ e.g. is found carpels in insteada place ofwhere stamens organs in ofthe another pistillata-2 typemutant are of normally Arabidopsis found,. e.g. carpels instead of stamens in the pistillata-2 mutant of Arabidopsis. Received: 2 May, 2009. Accepted: 21 April, 2010. Invited Review International Journal of Plant Developmental Biology 4 (Special Issue 1), 1-16 ©2010 Global Science Books Fig. 1 Genetic model of organ identity. (A) ABCDE model of floral organ identity in core eudicots; (B) flower of Arabidopsis thaliana (Brassicaceae); (C) flower of Antirrhinum majus (Plantaginaceae); (D) “floral quartet” model of organ specification (in Arabidopsis, modified after Theissen and Saedler 2001; Zahn et al. 2005a); this model directly links floral organ identity to the presumed action of five different tetrameric transcription factor complexes of MADS-box proteins (termed as a, b, c, d, e); (E+G) modifications of the ABC model: “sliding-boundary-model”; (E) modified ABC model for most monocots; (F) flower of Tulipa bakeri (Liliaceae); (G) modified ABC model for Rumex acetosa (Polygonaceae); (H) flower of Rumex acetosa (Polygona- ceae); (I) ABCDE model for basal angiosperms: “fading-borders” model; (J) flower of Illicium anisatum (Illiciaceae). – A, B, C, D, E: domains of action of the floral homeotic genes; a, b, c, d, e: interaction of the proteins (transcription factors); Ca = carpel, P = petal, S = sepal, St = stamen, Ov = ovule. three classes of homeotic genes, either acting alone or in box.2 The resulting ABCDE proteins bind to the target combination. Expression of class A genes specifies sepal genes in complexes larger than a dimer (Egea-Cortines et al. formation. The combination of class A and B genes spe- 1999; Honma and Goto 2001). Thereby, they activate or cifies the formation of petals. Class B and C genes specify repress target gene expression as appropriate for the deve- stamen formation and expression of the class C gene alone lopment of the different floral organ identities. Thus the determines the formation of carpels. A second major tenet “quartet model” of floral organ specification (Fig. 1D) of the ABC model is that A and C activities are mutually postulates that five different tetrameric transcription factor repressive. In 2001, the classical ABC model was extended complexes composed of MADS proteins specify floral to the ABCDE model (Theissen 2001; Fig. 1A). D function organ identities (Theissen 2001; Theissen and Saedler 2001; genes are required for ovule development (e.g. Colombo et Ferrario et al. 2003; Kaufmann et al. 2005; Zahn et al. al. 1995, Angenent and Colombo 1996) and at least one of 2005a). The “quartet model” is a molecular model – the E genes is required for the organ identity in all five although in some aspects still highly hypothetical – that organ categories (e.g. Pelaz et al. 2000; Zahn et al. 2005a, advances the genetic ABCDE model and directly links flo- 2005b; for further citations see Erbar 2007). ral organ identity to the action of MADS-domain proteins. The interaction of these five different gene classes is shown in the so-called floral quartet model. The genes A, B, C, D, and E encode transcription factors, i.e. proteins that 2 MADS is an acronym derived from the founding four members